Analytical tools for efficient extraction, preconcentration and detection of catecholamines (dopamine, epinephrine and norepinephrine) in biological matrices (such as urine) are important from a clinical point of view. Catecholamines have been investigated as potential biomarkers for the diagnosis and monitoring of tumors associated with different types of cancers and neural disorders [1]. Excess production of catecholamines by these tumors can cause “hypercatecholaminemia” which may cause health complications such as cerebrovascular accident, heart failure, cardiomyopathy and other potent impacts on the cardiovascular system [2]. Catecholamines are excreted in urine mainly in the following forms: deaminated metabolites, unchanged, and o-methylated amines (metanephrines). Analyzing catecholamines in urine, plasma or blood samples require sample preparation, preconcentration and cleanup steps essential for the minimization of any interfering components that might be present in biological matrices. In current practices, catecholamine sample pretreatments are predominantly performed by solid phase extraction (SPE) utilizing two types of sorbents: (a) polymeric reversed-phase resins (e.g., Oasis HLB from WATERS, and PLRP-SPE from Agilent) and (b) phenylboronic acid-functionalized silica particles. Polymeric sorbents are typically made of N-methylpyrrolidone and divinylbenzene monomers [3], and they possess excellent pH stabilities as well as balanced hydrophilic-hydrophobic characteristics. However, the low specific affinity toward the polar catecholamines can be enhanced through their chemical modification (derivatization). This is typically accomplished via formation of diphenylboronate-catecholamine complex [4] to facilitate their analysis by HPLC [5] or capillary electrophoresis [6]. A notable shortcoming is that extraction beds prepared from organic polymers possesses slow mass transfer characteristics analogous to the chromatographic stationary phases prepared from polymeric materials [7]. This may result in delayed or incomplete desorption of the extracted analytes from the sorbent bed causing sample loss and/or carryover problems.
The other type of extraction media used for the analysis of catecholamines is based-on silica particles with phenylboronic acid ligand (PBA-SPE). They have been widely used and commercialized by Agilent. Phenylboronic acid ligand has high affinity toward cis-diol groups present in the catecholamines [8]. The activation of the complexation ligand (phenylboronate, pKa ˜9.5 [9]) requires conditioning of the SPE cartridge with high-pH buffer (pH 10-12) [10] giving rise to the main drawback of PBA-SPE cartridges due to inadequate pH stability of silica-based particles known to have narrow operational pH window (pH 2-8) [11-14].
Malik and coworkers [15-18] have developed a number of sol-gel extraction phases for capillary microextraction (CME) coupled to gas chromatography (GC) or high-performance liquid chromatography (HPLC) providing excellent pH stability (0.0˜14.0) using different metal/metalloid alkoxide precursors providing titania-[19,20], zirconia-[16], and germania-based [14,17,18] hybrid organic-inorganic coatings for capillary microextraction. Hydrolytic sol-gel (HSG) route [11] was used to create those microextraction media. Non-hydrolytic sol-gel (NHSG) route has been investigated extensively in the field of catalysis for the creation of metal/metalloid oxides [21,22]. In water-free environment, transition metal halide (e.g., ZrCl4) concurrently undergoes alcoholysis and condensation reactions leading to the formation of transition metal oxides [23]. NHSG transition metal oxides possess (a) high homogeneity, (b) more Lewis acid sites than Bronsted acid-base sites and (c) better water-tolerance [21,22,24-26]. NHSG route can provide uniformly dispersed transition metal oxide particles in organic solvents and allows surface modification with organic moieties [27-30]. The latter property is a crucial point for the use of NHSG route for the creation of hybrid organic-inorganic material with covalent bonding between the organic ligand and the transition metal oxide network. Described herein is the synthesis and analytical evaluation of a novel zirconia-based sol-gel hybrid organic-inorganic sorbent to provide a biocompatible extraction medium integrating amphiphilic properties with enhanced thermal-, mechanical- and pH stability characteristics for the analysis of aqueous samples containing free catecholamines and molecules structurally related to their metabolites (FIG. 1).